Avalanche Problems
The 8 avalanche problems used in forecasting. Understanding these helps you interpret forecasts and make better terrain decisions.
What are Avalanche Problems?
Avalanche forecasters organize conditions into these 8 problem types. Each forecast identifies which problems exist, where they're found (elevation/aspect), their likelihood, and expected size. This framework helps you match terrain choices to current conditions.
Storm Slab
New snow bonding poorly to the old snow surface or within the new snow itself. Fresh storm snow creates reactive slabs that are sensitive to human triggers.
Distribution
Widespread
Likelihood
Likely to Very Likely
Typical Size
Small to Large (D1-2.5)
Duration
Hours to days (during and shortly after storm)
Key Characteristics
- •Forms during and immediately after storms
- •Most reactive in first 24-48 hours
- •Can affect all elevations and aspects
- •Sensitivity decreases as snow settles
- •Often triggered by first person on slope
⚠️ Terrain to Avoid
- ✕Slopes steeper than 30°
- ✕Wind-loaded features
- ✕Convex rollovers
- ✕Unsupported slopes
✓ Safer Terrain
- ✓Dense trees
- ✓Low-angle terrain (<25°)
- ✓Ridgetops and windward slopes
- ✓Previously tracked slopes (with caution)
💡 Management Tips
- •Wait 24-48 hours after storm for stabilization
- •Stick to low-angle terrain during storms
- •Avoid lee slopes during active wind transport
- •Watch for rapid changes in snow quality
Wind Slab
Wind-transported snow forming cohesive slabs on lee slopes. Wind creates dense, stiff slabs that bond poorly to the underlying surface.
Distribution
Specific lee aspects and features
Likelihood
Likely
Typical Size
Small to Large (D1-2.5)
Duration
Days to weeks
Key Characteristics
- •Forms on leeward slopes (downwind)
- •Often has smooth, rounded appearance
- •Pillow-like features visible
- •Reactive to human triggers
- •Can persist after storm ends
⚠️ Terrain to Avoid
- ✕Lee slopes (downwind aspects)
- ✕Cross-loaded gullies and bowls
- ✕Convex terrain features
- ✕Areas below cornices
✓ Safer Terrain
- ✓Windward (upwind) slopes
- ✓Dense timber
- ✓Protected terrain
- ✓Ridgelines (with caution)
💡 Management Tips
- •Identify wind direction from forecast
- •Avoid lee aspects for 24-48 hours after wind
- •Look for wind-affected snow surface
- •Test small features before committing to larger slopes
Persistent Slab
Slab formed over a persistent weak layer that can remain sensitive for weeks or months. Often surface hoar, facets, or depth hoar.
Distribution
Specific elevations and aspects
Likelihood
Possible to Likely (very unpredictable)
Typical Size
Large to Very Large (D2-3)
Duration
Weeks to months
Key Characteristics
- •Can remain reactive for weeks to months
- •Unpredictable trigger locations
- •Large, destructive avalanches
- •Difficult to assess stability
- •Remote triggers and propagation common
⚠️ Terrain to Avoid
- ✕Slopes matching problem elevation/aspect
- ✕Terrain traps
- ✕Connected terrain (propagation risk)
- ✕Slopes above 30°
✓ Safer Terrain
- ✓Elevations/aspects not affected
- ✓Dense timber
- ✓Low-angle terrain
- ✓Well-supported slopes
💡 Management Tips
- •Conservative terrain choices
- •Avoid problem elevations/aspects entirely
- •No reliable stability tests
- •Wait for major weather event to reset snowpack
Deep Persistent Slab
Slab over a deep persistent weak layer (often depth hoar or facets). Extremely dangerous - rare to trigger but catastrophic when they release.
Distribution
Specific terrain and elevation
Likelihood
Unlikely but Possible (low frequency, high consequence)
Typical Size
Very Large to Historic (D3-5)
Duration
Months to entire season
Key Characteristics
- •Very large, terrain-scale avalanches
- •Extremely difficult to trigger
- •Catastrophic consequences when released
- •Cannot be managed by recreational users
- •Signs of instability rare before release
⚠️ Terrain to Avoid
- ✕Avoid entirely if problem exists
- ✕Slopes in problem elevation bands
- ✕Terrain traps
- ✕Large, connected terrain
✓ Safer Terrain
- ✓Elevations not affected
- ✓Dense timber (if depth hoar not present)
- ✓Very low-angle terrain
- ✓Different mountain ranges
💡 Management Tips
- •Avoid terrain entirely when problem exists
- •No safe way to assess or manage
- •Listen to forecaster warnings
- •Choose different objectives
Wet Loose Avalanche
Point release of wet snow that entrains more snow as it descends. Caused by water infiltrating and weakening the snowpack.
Distribution
Sun-exposed and warm aspects
Likelihood
Likely (predictable timing)
Typical Size
Small to Large (D1-2.5)
Duration
Diurnal cycle (daily warming)
Key Characteristics
- •Predictable timing (midday/afternoon)
- •Starts at a point, fans out
- •Visible rollerballs and pinwheels
- •Dense, heavy debris
- •Can entrain large amounts of snow
⚠️ Terrain to Avoid
- ✕Steep terrain during warming
- ✕Sun-exposed slopes in afternoon
- ✕Terrain traps (debris is dense)
- ✕Gullies and couloirs
✓ Safer Terrain
- ✓Shaded aspects
- ✓Low-angle terrain
- ✓Frozen snow (early morning)
- ✓Ridgelines
💡 Management Tips
- •Early morning starts
- •Descend before warming
- •Watch for pinwheels and rollerballs
- •Know when snow surface freezes/thaws
Wet Slab Avalanche
Cohesive slab of wet snow failing. Water weakens bonds within snowpack creating dangerous slab conditions.
Distribution
Varies - can be widespread or isolated
Likelihood
Possible to Likely (depends on warming)
Typical Size
Large to Very Large (D2-3.5)
Duration
Hours to days (warming periods)
Key Characteristics
- •Slower moving but very dense
- •Often full-depth avalanches
- •Can release spontaneously
- •Difficult to escape if caught
- •Debris sets up like concrete
⚠️ Terrain to Avoid
- ✕Steep slopes during prolonged warming
- ✕Terrain after rain-on-snow
- ✕South aspects in spring
- ✕Slopes showing water infiltration
✓ Safer Terrain
- ✓Low-angle terrain
- ✓Frozen snow (cold mornings)
- ✓Shaded aspects
- ✓Well-drained slopes
💡 Management Tips
- •Get off slopes when snow becomes wet
- •Early starts essential
- •Watch for free water in snowpack
- •Avoid if temperatures above freezing overnight
Glide Avalanche
Entire snowpack sliding on the ground. Glide cracks indicate movement but release timing is unpredictable.
Distribution
Smooth, grassy slopes and slabs
Likelihood
Possible (unpredictable timing)
Typical Size
Large to Very Large (D2-3.5)
Duration
Weeks to months
Key Characteristics
- •Full-depth to ground
- •Glide cracks visible before release
- •Timing completely unpredictable
- •Often releases spontaneously
- •Cannot be managed
⚠️ Terrain to Avoid
- ✕ANY slope with glide cracks
- ✕Below glide cracks (runout)
- ✕Smooth, grassy slopes in warming
- ✕All slopes with visible glide cracks
✓ Safer Terrain
- ✓Slopes without glide cracks
- ✓Rough ground surface (rocks/trees)
- ✓Different aspects
- ✓Low-angle terrain
💡 Management Tips
- •AVOID ALL SLOPES WITH GLIDE CRACKS - period
- •Give 100+ feet clearance
- •Cannot predict when they release
- •No safe way to assess or test
Loose Dry Avalanche
Non-cohesive dry snow sluffing. Usually small point releases but can entrain significant amounts of snow on steep terrain.
Distribution
Very steep slopes (>35°)
Likelihood
Likely on steep terrain
Typical Size
Small to Medium (D1-2)
Duration
During/after fresh snow
Key Characteristics
- •Starts at point, fans out
- •Non-cohesive snow
- •Occurs on very steep terrain
- •Can be triggered by skier/climber
- •Multiple sluffs can combine
⚠️ Terrain to Avoid
- ✕Extremely steep terrain (>40°)
- ✕Terrain traps below
- ✕Gullies that concentrate debris
- ✕Cliffs and exposure
✓ Safer Terrain
- ✓Lower-angle terrain (<35°)
- ✓Open slopes with runout
- ✓Protected aspects
- ✓Ridgelines
💡 Management Tips
- •Common and generally manageable
- •Avoid terrain traps below
- •One person at a time on steep faces
- •Watch for accumulating debris
Using Avalanche Forecasts
1. Check Daily
Read your local forecast every morning, even if not going out. Learn how forecasters describe conditions.
2. Identify Active Problems
Look for which of the 8 problems exist today. Forecasts list them in order of concern.
3. Note Elevation & Aspect
Pay attention to where problems exist. Sometimes different elevations have different problems.
4. Match Terrain to Problems
Avoid terrain where problems exist. Choose slopes that don't match problem parameters.
5. Reassess in Field
Forecasts are regional. Observe actual conditions and adjust plans if observations don't match forecast.